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    <!-- 
        join、slice()、indexOf和concat方法 不能改变数组的结构
        join(separatar)   //separatar：分隔符,分隔数组的每一项
    -->
    <script>
        var arr=[1,2,3]
        var str=arr.join("");
        console.log(str)     //123
        console.log(arr.join())   //返回原数组  1，2，3
        console.log(arr.join("/"))   //  1/2/3
    </script>
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